1. Field of the Invention
The present invention generally relates to a vibration control system for attenuating vibration input in steering equipped with a tilt mechanism for adjusting the vertical position of the steering wheel by means of tilting motion of a moveable column linked tiltably in the vertical direction to the distal end of a fixed column fixed to a vehicle body, and more specifically, relates to a vibration control system for an electric tilt steering system in which adjustment of steering vertical position is driven by an electric motor.
2. Description of the Related Art
Automobile steering has a cantilever support structure where a steering wheel is supported at one end, namely a fixed column end, on the body of the vehicle, and thus resonates in a low-frequency band of about 20–50 Hz corresponding to the idling frequency of a typical engine. Thus, the steering vibrates appreciably with vibratory force at this natural resonance frequency, resulting in the problem of vibration of steering during engine idling. One method for addressing such vibration of steering involves shifting the natural resonance frequency thereof into a higher frequency band than the low-frequency band mentioned above. According to this approach, mounting rigidity of the steering is made higher, or a dynamic damper is attached to the steering.
In recent years, steering systems equipped with electric tilt mechanisms or electric telescopic mechanisms for electrically-powered adjustment of the vertical and front/back positions of the wheel have become widespread. However, such electrically-powered mechanisms increase the mass of the steering, making it more difficult to raise the aforementioned natural resonance frequency. Additionally, in the case of steering equipped with a telescoping mechanism, since resonance frequency changes each time that a user makes positioning adjustments, a resultant problem is the difficulty of achieving vibration damping action with a single dynamic damper.
To cope with the above problem, JP-A-6-107186 discloses, for example, a modified dynamic damper wherein: an upper shaft has a conical circumferential face that decreases in diameter towards its end; a steering tube disposed about the upper shaft support an holder; and an elastic body is supported by the holder such that the elastic body has affixed to a first end thereof a sliding member that slides against the conical face and to the other end thereof a mass, in order to provide an effective deformation interval between the holder and the mass. According to this dynamic damper, the elastic body extends and retracts in association with vertical movement of the upper shaft so that the effective deformation interval changes in a manner proportional to the length of the steering shaft. Thus, the vibration frequency of the dynamic damper changes on the basis of change of the spring constant of the elastic body, whereby even if the length of the steering shaft should change, the vibration frequency of the dynamic damper will be adjusted automatically to correspond therewith.
Another device is a dynamic damper as described in JP-A-2003-40112, wherein the steering column or steering wheel is provided with a plurality of mutually independent dynamic dampers composed of mass members elastically supported by spring members. The natural frequencies of dynamic dampers are set such that at least one is present in a region of frequency variation range of the steering wheel in association with extension and retraction of the steering column, at both the low frequency end and high frequency end with the median value for the frequency variation range between them. This dynamic damper is able to provide advantageous vibration damping effect against changing natural vibration of the steering wheel in association with extension and retraction of the steering column, with a relatively small number of dynamic dampers installed.
However, the former dynamic damper stated above is mounted as a separate component onto the steering tube, and the latter dynamic damper described above is likewise mounted as a separate component onto the steering wheel. In either case, component costs are required and labor is needed to attach the component, so that higher steering cost becomes a problem.